This is my personal shop: http://www.spoonflower.com/profiles/rusticcorgi
They have a standard price on each fabric, if you buy your own design you get a discount off the base price of 10%, if someone buys a print you're selling you get 10% in profit.

And if you would like to make your own fabric you can upload your own images and have them printed. It's easier if you have a good image editing program to "clean up" art or resize photos, but you can just scan say your kids art onto the computer and upload it. There are actually quite a few how-to blog posts and YouTube videos for more instructions out there now. It's really a lot of fun.